© Copyright 2006 American Chemical Society
SEPTEMBER 12, 2006 VOLUME 22, NUMBER 19
Letters Controllable Preparation of Monodisperse O/W and W/O Emulsions in the Same Microfluidic Device J. H. Xu, S. W. Li, J. Tan, Y. J. Wang, and G. S. Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed March 2, 2006. In Final Form: July 21, 2006 This letter presents a simple way to prepare monodisperse O/W and W/O emulsions in the same T-junction microfluidic device just by changing the wetting properties of the microchannel wall with different surfactants. Highly uniform droplets ranging from 50 to 400 µm with a polydispersity index (σ) value of less than 2% were successfully prepared. With the change in surfactants and surfactant concentrations, the interfacial tension and the wetting properties varied, and disordered or ordered two-phase flow patterns could be controllable. Monodisperse O/W and W/O emulsions were prepared under the action of a cross-flowing shear force or a perpendicular shear force by using an oil solution with 0.1-2.0 wt % Span 80 and an aqueous solution with 0.1-2.0 wt % Tween 20 as a continuous-phase flow, respectively. It gives a controllable method of preparing O/W and W/O emulsions in the same microfluidic device.
Introduction Emulsions are important materials and products in the food, pharmaceutical, cosmetics, and chemical industries. Common emulsions include oil-in-water (O/W) and water-in-oil (W/O), namely, direct and inverted emulsions. The addition of a surfactant is essential for long-term stability because emulsions are thermodynamically metastable. Monodisperse emulsions have received a great deal of attention for various applications as a result of their improved stability and the facilitated control of their properties.1-3 Typically, emulsions are made by fissioning droplets with shear or impact; the resulting suspensions process a wide size distribution of drop sizes. Several strategies have been proposed to reduce the polydispersity of droplets.4,5 Recently, the application of micromaching techniques has grow rapidly in * To whom correspondence may be addressed. E-mail: gsluo@ tsinghua.edu.cn. Tel: +86-10-62783870. Fax: +86-10-62783870. (1) Nakashima, T.; Shimizu, M.; Kukizaki, M. AdV. Drug DeliVery ReV. 2000, 45, 47. (2) Omi, S. Colloids Surf., A 1996, 109, 97. (3) Nishisoko, T.; Torii, T.; Higuchi, T. Chem. Eng. J. 2004, 101, 23. (4) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600. (5) Umbanhowar, P. B.; Prasad, V.; Weitz, D. A. Langmuir 2000, 16, 347.
Figure 1. T-junction microfluidic device.
various fields, and new applications are appearing in biotechnology and chemical reactions, such as microanalysis, on-chip separation, and chemical microreaction.6-9 Monodisperse droplets in microfluidic devices have been generated via a number of methods, including geometry-dominated breakup,10-12 droplet breakup through through-hole array,13-15 cross-flowing rupturing (6) Song, H.; Ismagilov, R. F. J. Am. Chem. Soc. 2003, 125, 14613. (7) Chen, X.; Wu, H.; Mao, C.; Whitesides, G. M. Anal. Chem. 2002, 74, 1772. (8) Harrison, D. J.; Skelton, V. Trends Anal. Chem. 2000, 19, 389. (9) Zheng, B.; Roach, L. S.; Ismagilov, R. F. J. Am. Chem. Soc. 2003, 125, 11170. (10) Sugiura, S.; Nakajima, M.; Iwamoto, S.; Seki, S. Langmuir 2001, 17, 5562.
10.1021/la0605743 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
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Figure 2. Effect of surfactant concentration on the interfacial tension and contact angle.
Figure 3. Evolution of the flow patterns with the surfactants and their concentration at the T-junction section. The water phase was injected from the perpendicular capillary, and the oil phase was injected from the horizontal channel. The two-phase flow rates were both 20 µL/min.
fluid with the walls are exceedingly important parameters in determining whether ordered monodisperse droplets can be prepared, and the contact angle of the fluid with the channel wall could be controlled by adding surfactants of different concentrations. In this letter, we used the simple T-junction microfluidic device to prepare monodisperse O/W and W/O emulsions by changing the wetting properties of fluids with the wall. We investigated the effects of surfactants and their concentrations on contact angles between the oil/water phase and the channel wall. On the basis of this, we successfully prepared monodisperse O/W and W/O emulsions by using aqueous solution with 0.12.0 wt % Tween 20 and oil solution with 0.1-2.0 wt % Span 80 as the continuous-phase flow, respectively. Experimental Section
Figure 4. Micrographs of the oil-water interface at the intersection channel during the droplet formation process. The oil-phase flow rate was 50 µL/min, and the water-phase flow rate was 10 µL/min.
through a micropore or microchannels array,16,17 perpendicularflow-induced breakup,18 hydrodynamic flow focusing through a small orifice,19-21 and cross-flowing rupture in a T-junction microchannel.22-26 Alternatively, highly uniform emulsion droplets with standard deviations of less than 5% could be generated. In these devices, O/W emulsions were prepared in hydrophilic microchannels,13-15,17,20 and W/O emulsions, in hydrophobic microchannels.12,19,21-23,25 However, the controllable preparation of W/O and O/W emulsions in the same microfluidic device by simply changing the operating conditions has not been reported. In our previous work, we developed a new flow route, the so-called perpendicular-shear-force-induced droplet formation in T-junction microchannels.18 A quartzose capillary was embedded into the perpendicular channel as the water-phase flow channel. We discovered that the wetting properties of the (11) Link, D. R.; Anna, S. L.; Weitz, D. A.; Stone, H. A. Phys. ReV. Lett. 2004, 92, 054503/1. (12) Tan, Y. C.; Fisher, J.; Lee, A. L.; Cristini, V.; Lee, A. P. Lab Chip 2004, 4, 292. (13) Kobayashi, I.; Nakajima, M.; Chun, K.; Kikuchi, Y.; Fujita, H. AIChE J. 2002, 48, 1639. (14) Kobayashi, I.; Mukataka, S.; Nakajima, M. Langmuir 2004, 20, 9868. (15) Kobayashi, I.; Mukataka, S.; Nakajima, M. Langmuir 2005, 21, 7629. (16) Kawakatsu, T.; Kikuchi, Y.; Nakajima, M. J. Am. Oil Chem. Soc. 1997, 74, 317. (17) Xu, J. H.; Luo, G. S.; Chen, G. G.; Wang, J. D. J. Membr. Sci. 2005, 266, 121. (18) Xu, J. H.; Luo, G. S.; Li, S. W.; Chen, G. G. Lab Chip 2006, 1, 131. (19) Anna, S. L.; Bontoux, N.; Stone, H. A. Appl. Phys. Lett. 2003, 82, 364. (20) Xu, Q.; Nakajima M. Appl. Phys. Lett. 2004, 85, 3726. (21) Ward, T.; Faivre, M.; Abkarian, M.; Stone, H. A. Electrophoresis 2005, 26, 3716. (22) Thorsen, T.; Roberts, R.; Arnold, F.; Quake, S. Phys. ReV. Lett. 2001, 86, 4163. (23) Nisisako, T.; Torii, T.; Higuchi, T. Lab Chip 2002, 2, 24. (24) Dreyfus, R.; Tabeling, P.; Willaime, H. Phys. ReV. Lett. 2003, 90, 144505/ 1. (25) Tice, J. D.; Lyon, A. D.; Ismagilov, R. F. Anal. Chim. Acta 2004, 507, 73. (26) Cristini, V.; Tan, Y. C. Lab Chip 2004, 4, 257.
Microfluidic Device. A simple T-junction microfluidic device was used, which was fabricated on 100 mm × 20 mm × 5 mm polymethyl metacrylate (PMMA) using an end mill. The oil-phase flow channel dimensions were approximately 300 µm wide × 200 µm high. A quartzose capillary with an inner diameter of 50 µm was embedded into the perpendicular channel (200 µm wide × 200 µm high) as the water-phase flow channel. The measured channel dimensions were approximately 400 µm wide × 400 µm high (Figure 1). Two microsyringe pumps and two gastight microsyringes were used to pump the two phases into the microfluidic device, respectively. Materials. Anhydrous hexadecane was used as the oil phase. Deionized water was used as the water phase. Different concentrations of Span 80 used as the oil-soluble surfactants were added to the oil phase. Different concentrations of Tween 20 used as the watersoluble surfactants were added into the water phase. The surfactant concentration ranged from 0.0001 to 2.0 wt %. Apparatus and Analysis. The interfacial tension was measured by an interfacial tension meter using a spinning drop technique (XZD-3, China). Droplet formation experiments were carried out with a microscope at magnifications from 100× to 500×. A highspeed CCD video camera was connected to the microscope, and the images were recorded at a frequency of 200 images/s. The length of the plugs or the diameter of the droplets was measured from microscope images. After changing any of the flow parameter, we allowed at least 200 s of equilibration time. The average droplet size (dav) and the polydispersity index (σ) were determined by measuring the sizes of at least 100 drops from recorded pictures using homemade image analysis software. σ is defined by the following equation σ ) δ/dav × 100%, where σ is the polydispersity index, δ is the standard deviation, and dav is the average droplet diameter.
Results and Discussion Previous studies18,24 on oil-water two-phase flow in microchannels stated that the wetting properties of the liquids with respect to the wall surfaces deserved particular emphasis. The different types of surfactants and their concentrations could change the wetting properties of the microchannel wall. To examine how the surfactants and their concentrations affect two-phase flow properties in the T-junction microchannel device, the contact angle between fluids and PMMA was investigated with different surfactants.
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Figure 5. (a) Micrographs of monodisperse drops. The oil-phase flow rate was 200 µL/min, and the water-phase flow rate was 5 µL/min. The average droplet size dav is 98.5 µm, and the polydispersity index (σ) is 1.7%. (b) Effects of two-phase flow rates on the average droplet diameter. The continuous phase was a 2.0 wt % Span 80-hexadecane oil solution.
Figure 6. Micrographs of the oil-water interface at the intersection channel during the oil plug formation process. The oil-phase flow rate was 20 µL/min, and the water-phase flow rate was 50 µL/min.
First, we measured the contact angle of a deionized water drop, immersed in hexadecane (with different concentration of Span 80), in contact with the PMMA wall surface. In this experiment, we sampled a volume of 1 µL of water from a microsyringe to form a drop, gently deposited, on a clean PMMA surface. The cell was delicately filled with hexadecane afterward. We also measured the water-oil interfacial tension with the interfacial tension meter using the spinning drop technique. Figure 2a represents the interfacial tension and contact angle evolution with surfactant concentration Co. At low concentration, water experiences partial wetting with the PMMA surface. The contact angle is 82° and increases with increasing surfactant concentration. Above a concentration of 0.03% w/w, the contact angle sharply increases up to an angle close to 165°, and the wall surface becomes totally hydrophobic. However, the interfacial tension decreases with increasing surfactant concentration. When the surfactant concentration ranges from 10-4 to 0.03% w/w, the interfacial tension decreases from 58.0 to 3.2 mN/m, and the interfacial tension retains the minimum value at higher concentrations mainly because the critical micelle concentration (cmc) is close to 0.03% w/w for the working system. Therefore, we can change the PMMA surface from partially hydrophilic to completely hydrophobic by adding Span 80 at a concentration greater than that of cmc into hexadecane. Then, we measured the contact angle of a drop of hexadecane, immersed in water (with a different concentration of Tween 20), in contact with the PMMA surface using the same method. Figure
2b represents the interfacial tension and contact angle evolution with surfactant concentration Cw. As for what we expected, a similar evolution was observed. When the surfactant concentration is ranges from 10-4 to 0.1% w/w, the interfacial tension decreases from 58.0 to 4.8 mN/m, while the contact angle between the oil drop and the wall surface increases from 37 to 145°. For this system, the cmc value is close to 0.1% w/w, so we can also change the PMMA surface from oleophilic to oleophobic by adding Tween 20 at a concentration greater than that for cmc in the water phase. With respect to the experimental results and our current understanding of these phenomena for the given systems, complete wetting and leveling off of the interfacial tension are achieved when the systems approach the critical micelle concentration. We examined the effects of surfactants and their concentrations on the two-phase flow properties with different experimental systems, as shown in Figure 3. When there was no surfactant, disordered states were reached, and the oil and water phases partially adhered to the channel wall. When we added different concentrations of Span 80 to the oil phase, the evolution from disordered regimes to ordered water-dispersed flow could been seen. At lower Span 80 concentrations (0.05% w/w), well-defined water plugs/drops were formed, continuously moving with the oil phase flow. However, when we added different concentrations of Tween 20 to the water phase, the evolution from disordered regimes to ordered oil-dispersed flow could been seen. At lower Tween 20 concentrations (0.1%w/w), welldefined oil plugs were formed. Therefore, this is a new way to prepare O/W and W/O emulsions in the same microfluidic device by simply adding different surfactants to the water and oil phases, respectively.
Figure 7. (a) Micrographs of monodisperse drops. The oil-phase flow rate was 5 µL/min, and the water-phase flow rate was 200 µL/min. The average droplet size dav is 131.5 µm, and the polydispersity index (σ) is 1.35%. (b) Effects of two-phase flow rates on the average droplet diameter. The continuous phase was a 2.0 wt % Tween 20-water solution.
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Figure 4 gives four pictures of the water droplet formation process when we added 2.0 wt. % Span 80 to the oil phase as the continuous phase. The water drops were mainly ruptured by the oil cross-flowing shear force.Figure 5a shows the micrographs of monodisperse drops. The formed drops had a spherical shape of regular size. We investigated the effect of surfactant concentration on the average droplet size when the continuous phase contained different concentrations of 0.2, 0.5, and 2.0 wt % Span 80. The droplet diameter was independent of surfactant concentration mainly because that the interfacial tension reaches the minimum value when the surfactant concentration is more than cmc. We also investigated the effects of two-phase flow rates. The droplet size decreased with the increase in the continuous-phase flow rate under a certain dispersed-phase flow rate but slightly increased as the dispersed-phase flow rate increased (Figure 5b), which was similar to finding in many previous studies.22-26 Figure 6 gives four pictures of the oil plug formation process when we added 2.0 wt % Tween 20 to the water phase as the continuous phase. The water phase partially obstructed the oilphase flow at the junction but was not broken off at the channel interface as in traditional cross-flowing devices. Oil droplet formation was achieved by the water-phase shear force perpendicular to the oil-phase flow, generating microscale oil plugs/ drops, the so-called perpendicular shear-force-induced droplet formation process.18 Figure 7a shows the micrographs of monodisperse drops. Highly uniform oil droplets could also be prepared using the new droplet formation mechanism. Then, we investigated the effects of surfactant concentration and twophase flow rates on average droplet size. The droplet diameter was independent of surfactant concentration, decreased with the increase in the continuous-phase flow rate, and slightly increased as the dispersed-phase flow rate increased (Figure 7b). These results were similar to that of the above cross-flowing rupture process.
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From all of our studies, highly uniform water and oil droplets with polydispersity index (σ) values of less than 2% could be prepared, and the droplet size ranged from 50 to 500 µm.
Conclusions In this letter, we used a simple T-junction microfluidic device to prepare monodisperse O/W and W/O emulsions by changing the wetting properties of the microchannel walls with surfactants. We investigated the effects of surfactants and their concentrations on contact angles between oil and water phases and the channel wall. For the given systems, complete wetting and leveling off of the interfacial tension are achieved when the system approaches the critical micelle concentration. By adding different surfactants to the oil and water phases, the evolution from disordered regimes to ordered water-dispersed or oil-dispersed flow under the action of cross-flowing shear force or perpendicular shear force, respectively, could been observed. On this basis, we prepared O/W and W/O emulsions by using aqueous solutions with 0.12.0 wt % Tween 20 and oil solutions with 0.1-2.0 wt % Span 80, respectively, as continuous-phase flow and successfully prepared highly uniform droplets ranging from 50 to 500 µm with polydispersity index (σ) values of less than 2%. This is a controllable method of preparing O/W and W/O emulsions in the same microfluidic device. Acknowledgment. We acknowledge the Department of Biological Sciences and Biotechnology at Tsinghua University for providing the microfluidic device. We also gratefully acknowledge the support of the National Natural Science Foundation of China (20476050, 20490200, and 20525622) and SRFDP (20040003032) for this work. LA0605743
